Polyimide and the precursor are actively used for a base film of a printed circuit board, a cover film for a high integrated semiconductor device or a high integrated multilayer wiring board due to excellent durability, heat resistance, flame resistance, mechanical and electrical properties, and the like.
Recently, with thinning and high integration of FPC circuit, photoimageable coverlay/photosolder resist for a circuit board is used to improve fine circuit pattern and location accuracy by photolithography process, and relieve spring back of a bent. To be used as a photosensitive cover film for a circuit board, excellent durability, heat resistance, chemical resistance, plating resistance, mechanical and electrical properties are required.
In general, the photosensitive cover film for a circuit board is manufactured by heat pressing a liquid or film shaped photosensitive resin composition on a CCL (Copper Clad Laminate) circuit, exposing to UV according to patterns, developing with a developer, washing and drying, and heat curing, and fine holes required to connect the circuit to a desired location may be accurately made.
As the photosensitive protection film for a circuit board, a photosensitive resin composition that is prepared by adding acrylate, and the like to an epoxy resin used in the existing dry film is used. However, in this case, due to insufficient flame resistance and soldering heat resistance, resin may be discolored during soldering, or separated from the circuit, and due to insufficient flexibility and bending resistance, it cannot be applied for a part requiring repeated folding, and thus, it is limited for application as a photosensitive protection film for a circuit board.
Therefore, to overcome these problems, there is a need for a polyimide photosensitive resin having high heat resistance, bending resistance, and dielectric property, and being applicable for a protection film for the existing circuit pattern.
However, despite the necessity, there are several technical obstacles to use as a material for a polyimide photosensitive circuit protection film. Specifically, if polyimide is used as a photosensitive resin, it is used in the form of polyamic acid, which is a polyimide precursor favorable for moulding, and in this case, high temperature of 350° C. or more is required for polyimidation of polyamic acid.
In a PCB (Printed Circuit Board) process, the operation is progressed while coating or conjugating a protection film on a copper circuit having a pattern, and considering oxidation and deterioration of copper circuit, a process temperature maintains 200° C. or less. Thereby, since polyamic acid requiring a high temperature curing process of 350° C. or more has a limitation for application in the PCB process, a method for lowering a temperature of the polyamic acid curing process is required, and a lot of methods for lowering the curing temperature have been suggested in various literatures.
To lower a curing temperature, a polyamic acid precursor of thermoplastic polyimide having relatively low glass transition temperature (Tg) is used, or soluble polyimide that is already imidated is used.
However, in case where soluble polyimide is used, since imide itself is dissolved in a solvent, chemical resistance is insufficient and electroless gold plating property is not good, and in case where thermoplastic polyimide is used or siloxane is used, due to abnormal glass transition temperature (Tg), coefficient of thermal expansion rapidly increases and problems in terms of building stability and heat resistance was confirmed. If crosslinking density is increased in order to overcome these problems, low stiffness, which is the advantage of a photosensitive coverlay compared to a common coverlay, cannot be satisfied.